1. Technical Field
This invention relates to a paper or paper board coating composition containing a structured clay pigment and, more particularly, to such a coating that includes a structured clay pigment that increases the bulking characteristics and opacity of the coating without a detrimental effect on paper gloss.
2. Background Description
Fine particle size pigments, e.g. fine particle size kaolin clays, have been widely used in paper and paperboard coating formulations to provide desirable levels of gloss and opacity. However, such pigments often contain a substantial amount of ultrafine particles that are smaller than the wavelength of visible light and do not scatter light efficiently. Accordingly, such fine particle clay pigments yield lower coating opacity (also referred to as hiding power and contrast ratio) than clays of more optimal particle size.
In recognition of the foregoing, methods for increasing the light scattering ability of fine clay pigments have been developed. One such method involves structuring the clay pigment by chemical or thermal means. The objective in structuring a clay pigment is to provide microvoid pore structure via clay platelet aggregation and to aggregate the ultrafine particle fraction within the pigment (e.g., particles having a particle size of less than 0.2 microns) into structured particles of more optimal particle size with respect to light scattering efficiency. In high gloss applications, the chief technical challenge still confronting the use of clay structuring strategies is the ability to improve the physical and optical characteristics of the coating pigment without hurting sheet gloss.
Several techniques have been developed to form a structured clay from a fine particle clay feed to improve the opacity thereof. Examples of these techniques include calcination of fine particle clays, hydrothermal alteration of clay particles and chemical treatment of clay particles. Two common structured pigments used in paper coatings are structured calcined clays and synthetic alkali metal alumino-silicate (SAMS) compositions.
The calcination of fine particle clays at temperatures of close to 1000xc2x0 C. results in aggregation of the clay particle platelets, via thermal sintering, to form structured products that provide increased coating opacity. See, for example, U.S. Pat. Nos. 3,014,836 and 4,381,948.
U.S. Pat. Nos. 4,812,299, 4,863,796, 5,112,402, 5,186,746, and 5,316,576 each discloses a SAMS composition. SAMS pigments are structured clay compositions that are produced from the reaction of a kaolin clay with an inorganic base under hydrothermal conditions as generally disclosed in U.S. Pat. Nos. 4,812,299, 4,863,796, and 5,591,256.
It has been discovered that most calcined clays tend to be too abrasive to be used effectively in some paper-related applications. Further, existing calcined clays and SAMS compositions cannot be dispersed at high solids without structure loss and, therefore, exhibit poor high shear rheology. Moreover, such products are expensive to manufacture.
In recognition of the foregoing, methods of producing high performance, structured pigments with improved rheology and lower production costs have been developed. One such method involves treating the ultrafine particles in a kaolin (aluminosilicate) clay pigment with a flocculating agent in order to incorporate microvoid pore structure into the clay platelet aggregation. As stated above, the microvoids associated with structured pigments are responsible for providing desirable bulking and light scattering characteristics a pigmented coating. See, for example, U.S. Pat. Nos. 4,075,030, 4,076,548, and 4,078,941. These patents disclose increasing the opacifying power of kaolin clay pigments by flocculating the clay particles with a non-polymeric, polyamine aggregating agent at a low slurry pH. However, such structured pigments, when xe2x80x9cmade downxe2x80x9d into a slurry, do not have acceptable end-use properties for application in conventional paper coating compositions which are typically formulated at neutral to slightly alkaline pH values.
U.S. Pat. No. 4,738,726 discloses a method of treating a clay with a cationic flocculent in order to obtain a high bulking pigment with improved Theological properties. However, the Sedigraph mean particle size (i.e., the average Stokes Equivalent Particle Diameter) of the kaolin clay pigment disclosed is 0.5 to 0.6 microns, prior to treatment, with less than 35% by weight of particles finer than 0.3 microns. After flocculation the particle size is substantially increased. This substantial increase in particle size causes the structured pigment to be ineffective as a component in paper coatings when high gloss is required.
U.S. Pat. No. 5,676,748 discloses a method for producing bulking and opacifying kaolin pigments as wet-end fillers for paper. Specifically, an aqueous slurry of anionically dispersed kaolin particles is treated at low solids with high treatment levels of a low-to-medium weight cationic polymer which serves as a flocculent. The resultant aggregate structures contain little or no fine and ultra-fine particles and such structures contain a high number of internal voids or pores. However, once again, as a result of a drastic change in the particle size of the kaolin clay particles, the optical properties of the treated pigments are adversely affected when made down and added to a paper coating. Further, the structured clay pigments so provided can only be produced at low slurry solids (less than about 30%) which is unacceptable for use as a coating pigment in conventional paper coating applications.
U.S. Pat. No. 5,718,756 also discloses a method for manufacturing a structured paper coating which includes mixing a fine particle size kaolin clay pigment with a cationic flocculant, a nonionic binder and other additives. The cationic flocculant is added at a relatively high level and is thereby believed to create a relatively large increase in the mean particle size of the pigment. Further, while the rheological or optical properties of the resulting paper coating are not disclosed, the substantial increase in the particle size of the pigment is believed to hurt the glossing properties of the same.
The present invention is designed to overcome the deficiencies of the prior art discussed above. It is an object of the invention to provide a paper coating composition that includes a structured clay pigment that increases the bulking characteristics and opacity of the coating composition without having a detrimental effect on gloss characteristics of the same.
It is a further object of the invention to provide such a coating composition that includes a structured clay pigment that is utilized as a slurry product having a high percentage of solid mineral particles and desirable rheological properties.
In accordance with the illustrative embodiments and demonstrating features of the present invention, there is provided a coating composition for paper or paperboard products comprising a structured pigment. The structured pigment slurry includes a plurality of chemically treated mineral particles. The mineral particles have a Malvern median particle size of about 0.3 microns to about 3.5 microns. The Malvern median particle size of the treated particles is less than 20% greater than the Malvern median particle size of the mineral particles prior to treatment.
Other objects, features, and advantages will be readily apparent from the following detailed description of preferred embodiments thereof.